The CuA center of cytochrome c oxidase is a novel, cyclic Cu2S2 center, which serves as the primary electron receiver for the system. Electrons from reduced cytochrome c pass to the CuA, and in turn to cytochrome a and the binuclear cytochrome a3 - CuB center where dioxygen is reduced to water. We have solved the structures of CuA to 1.6 E by the MAD technique from data collected at SSRL 1.5, and the electron transfer partner cytochrome c552 to 1.4 E from data collected at SSRL 1.5 and 9.1. We now seek time to collect data on the reduced form of CuA as a means to beginning to understand the unique electron transfer properties of this center, and on crystals of the complex of CuA and cytochrome c552.